A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103, which in turn vibrate the oval window and round window openings of the cochlea 104. In response to received sounds transmitted by the middle ear 103, the fluid filled cochlea 104 functions as a transducer to generate electric pulses that are transmitted to the acoustic nerve 113, and ultimately to the brain. Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104.
In some cases, hearing impairment can be addressed by a cochlear implant that stimulates auditory nerve tissue with electrical stimulation signals delivered by multiple electrode contacts distributed along an implant electrode. FIG. 1 shows some components of a typical cochlear implant system where an external microphone provides an audio signal input to an external signal processing stage 111 that implements one of various known signal processing schemes. The processed signal is converted by the external signal processing stage 111 into a digital data format communication signal for transmission by a transmitter coil 107 into an implanted receiver stimulator 108. Besides extracting the audio information, the receiver stimulator 108 may perform additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through electrode wires 109 to an implant electrode 110. Typically, the implant electrode 110 includes multiple electrodes on its surface that provide selective stimulation of the cochlea 104.
Hearing systems such as those described above are known to have various difficulties associated with the transmitter coil 107. For example, in the arrangement described above, the transmitter coil 107 and the receiver stimulator 108 include permanent magnets used to hold the transmitter coil 107 in correct position over the receiver stimulator 108. These magnets create problems with magnetic resonance imaging (MRI) or at the skin which covers the receiver stimulator 108. In addition, the location of the transmitter coil 107 on the head behind the outer ear 101 leaves it exposed to impact (e.g., during sports) or being accidentally wiped or brushed off (e.g., while brushing the hair).
U.S. Pat. No. 4,696,287 by Hortmann et al. disclosed a coil system for an implanted hearing aid where the external transmitter coil was located in the ear canal with its windings wrapped perpendicular to the axis of the ear canal, and where the implanted receiver coil was a ring surrounding the ear canal. U.S. Pat. No. 7,120,501 by Boylston et al. disclosed a hearing implant system which included a transmitter coil in the ear canal and a receiver coil positioned in the middle ear on the other side of the ear drum from the transmitter coil. Both of the above systems required that the implanted receiver coil have wire windings concentrated at certain specific radii and no uniform winding from the center to an outer radius was possible. WO 9809588 by Seligman et al. disclosed a coil arrangement with one coil in the outer ear canal and another implanted beside the ear canal, where each coil included two orthogonally wound sub-coils around a ferrite core.